The present invention relates to a method for detecting an output short circuit in a switching regulator, and more particularly, to a method and circuit for detecting an output short circuit in a DC-DC converter.
Various types of power supply circuits are known in the prior art. For example, U.S. Pat. No. 7,292,016 describes a power supply circuit using a DC-DC converter, and Japanese Laid-Open Patent Publication No. 2003-33010 describes a multi-output power supply circuit implementing a short circuit detection function. FIG. 1 is a schematic circuit diagram of a conventional step-up type DC-DC converter 10 implementing a short circuit detection function. The DC-DC converter 10 includes first and second transistors T1 and T2, which are connected in series, and a PWM controller 20, which generates first and second drive signals DH and DL for respectively driving the first and second transistors T1 and T2. A coil L is connected to a node Nsw between the two transistors T1 and T2. Input voltage VIN is applied to the coil L. The coil L is connected to ground via a capacitor C1.
The first and second transistors T1 and T2 are each formed by an N-channel MOS transistor. The first transistor T1 includes a parasitic body diode D1 connected between its source (node Nsw) and drain. The second transistor T2 includes a parasitic body diode D2 connected between its source and drain (node Nsw). The drain of the first transistor T1 is connected to an output terminal 12, and the output terminal 12 is connected to ground via a capacitor C2. The source of the second transistor T2 is connected to ground.
The PWM controller 20 monitors output voltage OUT of the DC-DC converter 10 that is generated at the output terminal 12 to control the first and second transistors T1 and T2 in a complementary manner. When the second transistor T2 is activated (the first transistor T1 is deactivated), current energy is accumulated in the coil L, and current IL flows from the capacitor C1 via the coil L and the second transistor T2 to ground. When the first transistor T1 is activated (the second transistor T2 is deactivated), current IH, which is in accordance with the energy accumulated in the coil L, flows via the first transistor T1 to the output terminal 12. This increases the output voltage OUT. The ON/OFF ratio (drive duty) of the first and second transistors T1 and T2 is set at a value that is required for increasing the output voltage OUT from the input voltage VIN to a desired level.
The DC-DC converter 10 further includes a comparator 30, which compares the output current Iout with a short circuit detection threshold Dth to generate a short circuit detection signal Sdc. Based on the short circuit detection signal Sdc, the PWM controller 20 determines whether or not the capacitor C2 is short circuited, that is, whether or not an output short circuit is occurring. When an output short circuit is occurring, the PWM controller 20 deactivates the transistors T1 and T2.
FIG. 2 is a timing diagram illustrating the short circuit detection function of the DC-DC converter 10.
First, the second transistor T2 is activated, and current energy is accumulated in the coil L. At time ta, the first transistor T1 is activated and the second transistor T2 is deactivated. As a result, current IH, which is in accordance with the current energy accumulated in the coil L (the current IL when the activated transistor is switched), flows to the output terminal 12. The current IH charges the capacitor C2 and increases the output voltage OUT. This gradually decreases the current IH. Thereafter, the first and second transistors T1 and T2 are alternately activated at times tc, td, and te. This keeps the output voltage OUT constant.
When an output short circuit occurs, the output current Iout increases in a proportional manner as shown by the broken line in FIG. 2. More specifically, when the first transistor T1 is deactivated, some of the current IL flows to ground via the body diode D1 of the first transistor T1 and the short circuited capacitor C2. When the first transistor T1 is activated, the current IH flows to ground via the first transistor T1 and the capacitor C2. Accordingly, even after the first transistor T1 is activated, the output current Iout continues to increase. The comparator 30 constantly checks whether or not the output current Iout has exceeded the short circuit detection threshold Dth. For example, in FIG. 2, the output current Iout exceeds the threshold Dth at time tb. In this case, the comparator 30 generates the short circuit detection signal Sdc with a high level. In response to the short circuit detection signal Sdc, the PWM controller 20 determines that an output short circuit is occurring.
However, the short circuit detection function of the conventional DC-DC converter 10 has a shortcoming in that noise may cause erroneous detections. This problem will now be discussed in detail.
To protect a load that is connected to the DC-DC converter 10 from excessive current (short circuit current), it is preferable that the short circuit detection threshold Dth be set to a value that is close to the peak current Imax of the first transistor T1 (refer to FIG. 2). Thus, there is only a slight difference between the short circuit detection threshold Dth and the peak current Imax, and the detection margin Md provided for the short circuit detection operation is small. The small detection margin Md may result in a short circuit being determined as occurring even though there is actually no short circuit. For example, if the first transistor T1 is activated at time td, noise may cause the current IH flowing to the first transistor T1 to become greater than or equal to the short circuit detection threshold Dth. In such a case, the comparator 30 would generate the short circuit detection signal Sdc with a high level. Thus, although a short circuit is actually not occurring, the PWM controller 20 would determine that a short circuit is occurring. To prevent erroneous detection, the coil L may be enlarged. Enlargement of the coil L would decrease the speed in which current energy is accumulated in the coil L, that is, decrease the inclination of the current IL shown in FIG. 2. This would reduce fluctuations in the current IH caused by noise when the first transistor T1 is activated. However, enlargement of the coil L would increase the circuit scale. Accordingly, enlargement of the coil L would be impractical.